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Full text of "Wet water for forest fire suppression"

Historic, archived document 

Do not assume content reflects current 
scientific knowledge, policies, or practices. 



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FOREST RESEARCH NOTES 

U. S.DEPARTMENT OF AGRICULTURE 
FOREST SERVICE 

CALIFORNIA FOREST AND RANGE 
EXPERIMENT STATION * 

STEPHEN N. WYCKOFF, Director 




No. 71 



June 27, 1950 



WET WATER FOR FOREST FIRE SUPPRESSION^/ 

By 

Wallace L. Fons , Mechanical Engineer 



In recent years wetting agents have been recommended by manu- 
facturers and others for increasing the efficiency of water for fire 
suppression. Many local fire-protection agencies have made tests 
comparing wetting agent solutions with plain water for flame suppres- 
sion and mop-up under field conditions. In general, their results and 
conclusions indicated the need for comprehensive tests to determine 
the types of wetting agents best suited for suppression and the best 
technique for application of chemically treated water, called wet water. 

Such comprehensive research was begun in 1948 by the Engineering 
Department at the University of California at Los Angeles. Technical 
supervision of the research was administered by the Division of Forest 
Fire Research, California Forest and Range Experiment Station. In 1949 
the Station took over all phases of the investigative work. 

At the outset it was realized that a great deal of fundamental 
research would be necessary before specifications on all phases of 
suppression could be developed. For instance, those physical properties 
of water and chemicals accounting for the suppression action were not 
known. The research program was therefore divided into three parts: 

(1) Determining the physical properties of plain water and wet water; 

(2) investigating the mechanisms of fire suppression; (3) determining 
the suppression effectiveness of wet water over plain water. 



1/ Presented at: Annual meeting, Southern California Association 
of Foresters and Fire Wardens, San Bernardino, Calif., April 7, 1950, and 
Annual meeting, California Rural Fire Association, Merced, Calif., April 
21, 1950. 



* 



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Physical Properties 



The physical properties thought to be important in suppression 
are: surface tension, penetration, surface spreading, and foaming. 
Surface tension is a measure of the force required to hold a liquid 
drop in a spherical form. For instance, mercury has a high surface 
tension value (513 dynes/cm); water, a moderate value (72.8 dynes/cm); 
and alcohol and petroleum, low values (approximately 25 dynes/cm). Most 
wetting agents in concentration of one percent will reduce surface tension 
of water to 30 dynes/cm. The more active the wetting agent is, the more 
rapidly the surface tension of water is decreased. The recommended con- 
centration for fire fighting is usually such as to reduce the surface 
tension of water to about 35 dynes/cm. 

When the surface tension of water is reduced, spreading , penetrating, 
and foaming are increased. Tests of these physical properties were made 
on water ana 14 commercial wetting agents. The tests show that: 

1. Surface spreading of wet water on wood is 2 to 8 times 
greater than water, depending on species of wood. 

2. Penetration into wood of wet water is about 8 times 
greater than water. 

3. Penetration of wet water into charcoal is about 2/3 as 
much as it is for wood. 

4. Penetration and surface spreading of water on charcoal 
is practically nil. 

5. All wet water readily forms foam. 

Corrosion tests with iron and galvanized iron shewed that wet- 
ting agent solutions in general, unless they contain an inhibitor . 
have higher corrosion rates than distilled water. With few exceptions 
the rate of corrosion was greater for the galvanized iron. The corro- 
sion of metals by wetting agent solutions may be wholly corrected by 
the addition of small amounts of chemicals, known as corrosion inhibitors, 
such as potassium dichromate and sodium nitrite. Tests showed that as 
little as 100 parts per million, or about 2 ounces per 100 gallons of 
water, of commercial granular potassium dichromate was effective in 
reducing the corrosion of iron and galvanized iron by wetting agent 
solutions . 

Suppression Mechanism 

Before tests with fires were begun, some important questions 
arose concerning the suppression action of water First, what is the 
behavior of a water droplet when it strikes a burning wood surface? 
Calculations revealed that water, sprayed on burning wood with its 
surface reduced to charcoal, will impinge on the charcoal surface 
having a temperature as high .as 1900° F. Second, what is the 
theoretical minimum volume of water necessary to cool a unit volume 



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of burning wood below the rekindling point? Again, calculations showed 
that a unit volume of water suddenly applied over the entire surface 
will cool 300 volumes of burning wood below the kindling point; but if 
the water is applied only to the frontal area of the burning wood, the 
amount of water required is doubled. Third, does penetration cooling 
have an advantage over surface cooling and vice versa? Results of cal- 
culations indicated that there was no significant difference between 
penetration and surface cooling. 

Suppression Effectiveness 

Water may be used to an advantage in forest fire suppression 
for: (l) Flame suppression; that is, application of water on the flame 
to retard the spread of a given section of fire; (2) mop-up; that is, 
application of water on a burned area to extinguish small isolated 
flames and to cool glowing and charred material to a point at which 
rekindling is unlikely; (3) pretreatment ; that is, application of water 
on unburned fuels in advance of an oncoming fire to retard its rate of 
spread or in holding a line from which to backfire. For each of these 
uses, fire tests were conducted to compare the suppression effectiveness 
of wet water ana plain water. 

Flame Suppression 

For flame suppression, a total of 93 model fires were burned on 
which the effectiveness of 14 commercial wetting agents were tested. 
Besides these laboratory tests, 66 field-fire tests were made, in which 
three commercial wetting agents were tested. Each fire was allowed to 
burn up to its maximum flame intensity before suppression was started. 
The intensity of each fire was measured with a radiometer. From the 
radiometer record of each fire, the intensity during suppression was 
calculated . 

The experimental work established the superiority of wet water 
over plain water in reduction of fire intensity. The superiority is 
at its maximum when the liquid is first sprayed on the fire. Results 
indicate that this superiority at the very beginning of spray applica- 
tion on a section of flaming front is from three to four times greater 
than at the end of application, when only isolated flames remain. For 
the period of flame suppression of these experimental fires, the mean 
superiority values of wet water over plain water are 1.70 for field 
fires and 2.10 for model fires. Ordinarily, superiority is expressed 
as a ratio of quantity of plain water used to quanitty of chemical 
solution used. The superiority of wet water based on quantities was 
1.25 for field fires and 1.55 for model fires. 

However, in recommending wet water for flame suppression emphasis 
is given to the superiority based on reduction of flame intensity. The 
practical importance of this superiority, especially at the beginning 
of suppression, is that it shows the decided advantage a hose operator 
has in advancing on a fire front with wet water. This advantage should 
permit an operator to knock down the flame intensity more quickly to a 
point where he can move in and hold the spread of a fire. It is believed, 



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therefore, that the use of wet water in flame suppression should result 
in controlling a greater number of fires as well as in controlling the 
fires at an earlier period, thus reducing the size of the mop-up job. 
Even where an abundant supply of water is available, use of wet water 
in some instances may make the difference between success and failure 
in controlling a forest fire. 

In these tests the ratios of volume of burning "wood to volume of 
wet water used for flame suppression were: field fires, 56 to 1; model 
fires, 117 to 1. For fires suppressed with water the ratios were 45 to 
1 for field fires and 76 to 1 for model fires. These ratios, when com- 
pared with 300 tc 1 — the maximum possible, show that there is room for 
improvement in techniques of application. 

Experimental work has not yet advanced sufficiently to specify 
what pressures and discharge rates to use for most efficient flame 
suppression. It has been noted, however, that on the experimental 
fires sprays were superior to straight streams primarily because of 
their greater coverage. 

Mop-up 

To test the effectiveness of wet water in mopping up fires in 
deep pine litter, a total of 108 test fires were burned. The technique 
used in mopping up these fires consisted of first spraying the burned 
area as rapidly as possible, suppressing all glowing, smoking, and burn- 
ing material; yet not wasting water or wet water by wetting the area 
unnecessarily. In this first application care was taken to hold the 
nozzle above the surface of the burning litter at a distance which gave 
minimum air entrainment and effective spray distribution with maximum 
coverage. The first application was followed by an intermittent appli- 
cation on those spots where smoke appeared. A quick--opening-and-closing 
shut-off nozzle permitted closing the nozzle quickly when moving from one 
smoldering spot to another and opening it only long enough to extinguish 
the immediate small hot area. 

The results of 82 of these fires show that the superiority based 
on quantity of plain water used compared to quantity of wet water used 
is about 1.29. Thus, by the addition of wetting agents at appropriate 
concentration, a saving of 23 percent in the quantity of water can be 
realized on a given mop-up job. As intermittent application "was used part 
of the time in mopping up these fires, the superiority of wetting agent 
solutions based on time of mop-up is only 1.15. This means a saving of 
13 percent in time in mopping up a given burn. In practice the saving of 
23 percent in quantity of water and 13 percent in mop-up time may appear 
as not being significant; however, on a national scale or even on one lar£ 
burn such a saving can amount to a considerable sum of money. 

Another factor of practical importance is that wet water In mop-up 
has a distinct advantage in reducing the rekindling from hot spots and 
glowing embers. For the mop-up tests in deep pine litter the ratio of 
rekindled spots on plots suppressed with water to the rekindled spots on 
plots suppressed with wet water was 1.43 tc 1. For comparable fuel types 
one can expect approximately 30 percent fewer rekindlings on an area mopped 
up with wet water. 



Pretreatment 



Pre treatment experiments were made by spraying wet water and 
plain water on litter fuels to determine whether wet water would 
increase the length of time that wetting is effective. The results 
of these experiments indicate that the length of time for sprayed 
unburned litter fuels to dry is 50 percent longer for wet water than 
for plain water. This is attributed to the greater surface spreading 
and penetration of wet water. 

Conclusi ons 



Significant differences were found in physical properties among 
the wetting agent solutions tested. However , no special meaning can 
be attached to these differences because these same agents were not 
significantly different in their effectiveness in the fire suppression 
tests. It appears that the surface spreading, penetrating, and foaming 
properties of all the wetting agents tested, when the agent is used so 
as to reduce the surface tension of plain water by one-half, are within 
that range which does not alter the effectiveness of a wetting agent 
solution for fire suppression. The results indicate that on the basis 
of penetration and surface spreading, any one of the hundreds of wetting 
agents now on the market, if used at the proper concentration, would 
yield suppression-effectiveness results comparable to those obtained 
for the wetting agents tested. 

Specific conclusions drawn from work performed to date are: 
(l) Savings up to 2 3 percent in the volume of water required and 13 
percent in time of mop-up can be obtained with wet water if applied 
with reasonable efficiency; (2) rekindling is reduced as much as 30 
percent on fires mopped up with wet water as compared with plain water; 
(3) foaming appears to be a desirable property of wet water in mop-up 
because it prevents channeling of the water; (4) wet water is markedly 
superior to plain water in its ability to knock down flames quickly, 
thus permitting access to a fire edge not otherwise accessible; (5) 
dead fuels along the burning edge of a fire or along a backfire line 
remain wet up to 50 percent longer when sprayed with wet water than 
when sprayed with plain water; (6) fuels once treated with wet water 
and allowed to dry may be sprayed later with plain water with results 
comparable to an original spraying with wet water; (7) most wetting 
agents increase the corrosive action of water, but this can be wholly 
corrected by the addition of chemicals known as corrosion inhibitors. 

In general, the results to date have shown that wetting agents 
have a definite place in forest fire suppression. Certain information 
on foaming and dermatological effects, however, is still needed to 
permit the writing of definite specifications for the most desirable 
type of wetting agents. Some advancement has been made on technique 
of application for maximum effectiveness; but this knowledge is not 
yet complete enough to satisfy all field conditions. Experiments are 
still needed to determine the effect of form and pattern of spray on 
fire suppression efficiency, for plain water, wetting agent solutions, 
and other chemicals. 



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